The blast targeted members of the minority Shiite Muslim sect who were leaving the mosque when the bomb went off.

There was no immediate claim of responsibility but Sunni militant groups who do not consider Shiites to be true Muslims have carried out such attacks in the past.

A day of mourning was declared for Monday in the city, Pakistan's economic hub and the scene of years of police, sectarian and ethnic violence.

Initial reports suggested the bomb was rigged to a motorcycle, but top police official Shabbir Sheikh said later that an estimated 100kg of explosives was planted in a car.

Colonel Pervez Ahmad, of a Pakistani paramilitary force called the Rangers, said a chemical used in the explosion caught fire and spread the destruction beyond the blast site. Several buildings nearby were engulfed in flames.

Men and women wailed and ambulances rushed to the scene where residents tried to find victims buried in the rubble of collapsed buildings. The blast left a crater that was two yards wide and more than 4ft deep.

A top government official, Taha Farooqi, said at least 37 people were confirmed dead and 141 more wounded.

Sunni militant groups have stepped up attacks in the past year against Shiite Muslims who make up about 20% of Pakistan's population of 180 million people. Sunni militants linked to al Qaida and the Taliban view Shiites as heretics.